When a quantitative characteristic is subjected to natural or artificial selection, it is likely to change with the passage of time, provided that there is genetic variation for that characteristic in the population. Suppose that a dairy farmer wants to increase milk production among the cows in his herd. Variation at several loci potentially affects milk production in cows; some alleles at these loci confer high milk production, whereas other alleles confer low milk production. The dairy farmer breeds only those cows in his herd that have the highest milk production. If there is genetic variation in milk production (i.e., there are different alleles at the loci that control milk production), the mean milk production in the daughters of the selected cows should be higher than the mean milk production of the original herd. This increased production is due to the fact that the selected cows possess more alleles for high milk production than does the average cow, and these alleles are passed on to the offspring. Thus, the offspring of the selected cows possess a higher proportion of alleles for high milk production, and therefore produce more milk, than the average cow in the initial herd.

The extent to which a characteristic subjected to selection changes in one generation is termed the response to selection. Suppose that the average cow in a dairy herd produces 80 liters of milk per week. A farmer selects for increased milk production by breeding the highest milk producers, and the female progeny of those selected cows produce 100 liters of milk per week on average. The response to selection is calculated by subtracting the mean phenotype of the original population (80 liters) from the mean phenotype of the offspring (100 liters), obtaining a response to selection of 100 − 80 = 20 liters per week.

FACTORS INFLUENCING RESPONSE TO SELECTION The response to selection is determined primarily by two factors. First, it is affected by narrow-sense heritability, which largely determines the degree of resemblance between parents and offspring. When narrow-sense heritability is high, offspring will tend to resemble their parents; conversely, when narrow-sense heritability is low, there will be little resemblance between parents and offspring.

The second factor that determines the response to selection is how much selection there is. If the farmer is very stringent in the choice of parents and breeds only the highest milk producers in the herd (say, the top 3 cows), then all the offspring will receive genes for high milk production. If the farmer is less selective and breeds the top 20 milk producers in the herd, then the offspring will not carry as many genes for high milk production, and on average, they will not produce as much milk as the offspring of the top 3 producers would. The response to selection thus depends on the degree to which the selected parents differ from the rest of the population; this difference is measured by the selection differential, defined as the difference between the mean phenotype of the selected parents and the mean phenotype of the original population. If the average milk production of the original herd is 80 liters and the farmer breeds cows with an average milk production of 120 liters, then the selection differential is 120 − 80 = 40 liters.

CALCULATION OF RESPONSE TO SELECTION The response to selection (R) depends on narrow-sense heritability (h²) and the selection differential (S):

R = h² × S    (17.10)

This equation can be used to predict the magnitude of change in a characteristic when a given selection differential is applied. G. A. Clayton and his colleagues estimated the response to selection that would take place in the abdominal bristle number of Drosophila melanogaster. By using several different methods, they first estimated the narrow-sense heritability of abdominal bristle number in one population of fruit flies to be 0.52. The mean number of bristles in the original population was 35.3. They selected individual flies with a mean bristle number of 40.6 and intercrossed them to produce the next generation. The selection differential was 40.6 − 35.3 = 5.3; so they predicted a response to selection to be

R = 0.52 × 5.3 = 2.8

The response to selection of 2.8 is the expected increase in the characteristic in the offspring above the mean of the original population. They therefore predicted that the average number of abdominal bristles in the offspring of their selected flies would be 35.3 + 2.8 = 38.1. Indeed, they found an average bristle number of 37.9 in these flies.

ESTIMATING HERITABILITY FROM RESPONSE TO SELECTION Rearranging Equation 17.10 provides another way to calculate narrow-sense heritability:

In this way, h² can be calculated by conducting a response-to-selection experiment. First, the selection differential is obtained by subtracting the population mean from the mean of selected parents. The selected parents are then interbred, and the mean phenotype of their offspring is measured. The difference between the mean of the offspring and that of the initial population is the response to selection, which can be used with the selection differential to estimate the heritability. Heritability determined by a response-to-selection experiment is usually termed realized heritability. If certain assumptions are met, realized heritability is identical to narrow-sense heritability.

One of the longest-running selection experiments is a study of oil and protein content in corn seeds (Figure 17.14). This experiment began at the University of Illinois on 163 ears of corn with an oil content ranging from 4% to 6%. Corn plants with high oil content and corn plants with low oil content were selected and interbred. Response to selection for increased oil content (the upper line in Figure 17.14) reached about 20%, whereas response to selection for decreased oil content reached a lower limit near zero. Genetic analyses of the high- and low-oil-content strains revealed that at least 20 loci take part in determining oil content, one of which we explored in the introduction to this chapter. TRY PROBLEM 23

CONCEPTS